1. Field of the Invention
The invention relates to a battery, and more particularly to a battery structure provided with a flat wound electrode body that has been wound in a flat shape.
2. Description of the Related Art
In recent years, the importance of lithium ion batteries, nickel-hydrogen batteries, and other secondary batteries has increased in applications to power sources for vehicles or power sources for personal computers and portable terminals. In particular, lightweight lithium ion batteries, which make it possible to obtain a high energy density, are expected to be advantageously used as high-capacity power sources for vehicles.
A battery provided with an electrode body having a wound structure (wound electrode body) is a typical example of such a high-capacity lithium ion battery. In the battery of this type, a positive electrode sheet and a negative electrode sheet are disposed opposite each other via a separator sheet, and the resultant laminate is wound, thereby increasing the reaction surface area of the positive and negative electrodes. As a consequence, energy density can be increased and capacity can be raised.
When a battery is configured, the above-described wound electrode body is sometimes accommodated in an angular battery case (typically a flat box-like case). With such a configuration, the wound electrode body is pressed from two sides to mold in a flat shape and then inserted in the angular case. For example, Japanese Patent Application Publication No. 2007-242519 (JP-A-2007-242519) discloses a battery in which a tape that prevents unwinding is attached on the outermost periphery of a wound electrode body so as to prevent the wound electrode body from unwinding when the wound electrode body is press molded in a flat shape. Examples of other patent documents that cover technology relating to flat wound electrode bodies include Japanese Patent Application Publication No. 2005-222887 (JP-A-2005-222887), Japanese Patent Application Publication No. 2004-47332 (JP-A-2004-47332), and Japanese Patent Application Publication No. 2007-26939 (JP-A-2007-26939).
However, the aforementioned tape that prevents unwinding is designed to inhibit the unwinding of the outermost periphery of the flat wound electrode body, and an appropriate surface pressure applied to the wound electrode body is difficult to maintain by attaching the tape that prevents unwinding. Thus, where the flat wound electrode body expands during charging, a pressure produced by the electrode body expansion is particularly high at the winding end portions of the positive electrode sheet and negative electrode sheet (end positions on the outer periphery side in the longitudinal direction of the sheet; same hereinbelow), but the attached tape that prevents unwinding cannot apply a pressure acting in the expansion direction of the wound electrode body and the winding end portions of the positive electrode sheet and negative electrode sheet become loose.
FIG. 10 shows an example of a principal cross section of a conventional flat wound electrode body 1. In this configuration, a negative sheet 2 is disposed so as to be wound on the outer peripheral side with respect to a positive electrode sheet 3. A winding end portion 4 of the negative electrode sheet 2 and a winding end portion 5 of the positive electrode sheet 3 are disposed in the same curvature portion (rounded portion) 6 of the flat wound electrode body 1. Where the flat wound electrode body 1 expands during charging, the winding end portion 4 of the negative electrode sheet 2 and the winding end portion 5 of the positive electrode sheet 3 can become loose, and if the winding end portion 5 of the positive electrode sheet 3 located on the inner peripheral side is loose, because the surface pressure in the opposing portions of the negative electrode sheet 2 and the positive electrode sheet 3 is uneven, an electrode reaction proceeds unevenly between the positive electrode and negative electrode. Such an uneven electrode reaction between the positive electrode and negative electrode is undesirable because battery life in cycle endurance can be shortened.